US8208522B2ActiveUtilityPatentIndex 77
System and methods for receiving OFDM symbols having timing and frequency offsets
Est. expiryMar 7, 2028(~1.7 yrs left)· nominal 20-yr term from priority
H04L 2027/0067H04W 28/06H04L 27/2666H04L 2027/0095H04L 27/2659H04N 21/6112H04N 21/4382H04L 27/2662H04L 27/2675H04L 27/2678H04N 21/2383H04L 2027/003H04L 27/2607H04L 27/2613
77
PatentIndex Score
7
Cited by
16
References
35
Claims
Abstract
Systems and methods for receiving an OFDM preamble without knowledge of channel characteristics are provided. An OFDM preamble signal with frequency shifted cyclic extensions is received. Taken together the cyclic extensions form a frequency shifted version of the OFDM preamble signal. Frequency offsets and timing offsets are estimated and corrected in an efficient manner using a simple concatenation approach in the time domain, followed by a summation of the OFDM preamble signal and the concatenation after a transformation of the OFDM preamble and the concatenation into the frequency domain. Phase errors in the frequency domain are estimated and corrected after FFT transformations of the received signals. A valid preamble is detected and additional parameters for receiving subsequently transmitted OFDM symbols in a channel are extracted from the OFDM preamble. The methods are computationally efficient and robust. Receiver implementations for performing the methods in a DVB receiver are disclosed.
Claims
exact text as granted — not AI-modified1. A method, comprising:
receiving an orthogonal frequency division multiplexed preamble comprising symbol A of length N samples, a prefix C comprising a frequency shifted cyclic extension of a first portion of symbol A of length N/2 samples plus an offset K samples, and a suffix B comprising a frequency shifted cyclic extension of a remaining portion of symbol A of length N/2 samples minus offset K samples;
concatenating prefix C and suffix B in the time domain to form a concatenated symbol CB that is a frequency shifted version of symbol A having length N; and
utilizing the concatenated symbol CB to estimate at least one of a phase rotation and a phase error.
2. The method of claim 1 , wherein utilizing the concatenated symbol CB to estimate at least one of a phase rotation and a phase error further comprises:
transforming symbol A into the frequency domain by performing a fast Fourier transform of length N; and
transforming the concatenated symbol CB into the frequency domain by performing a fast Fourier transform of length N.
3. The method of claim 2 , wherein utilizing the concatenated symbol CB to estimate at least one of a phase rotation and or a phase error further comprises:
estimating a phase rotation; and
correcting the phase rotation in the frequency domain.
4. The method of claim 3 , wherein estimating the phase rotation comprises estimating the phase error due to an integer frequency offset.
5. The method of claim 4 , wherein estimating the phase rotation due to an integer frequency offset comprises performing the calculation:
Δϕ
=
2
πɛ
N
C
N
where N C is the number of samples in prefix C, and ε is the normalized frequency offset.
6. The method of claim 3 , wherein correcting the phase rotation comprises:
performing a complex multiplication on the concatenated symbol CB to form a corrected symbol CB; and
coherently adding the symbol A transformed into the frequency domain and the corrected symbol CB.
7. The method of claim 2 , wherein utilizing the concatenated symbol CB to estimate at least one of a phase rotation and a phase error further comprises:
estimating a phase error due to a timing offset; and
correcting the phase error due to the timing offset during a demodulation in the frequency domain.
8. The method of claim 7 , wherein estimating the phase error due to the timing offset comprises performing the calculation:
ϕ
^
=
1
K
∑
k
∈
CDS
arg
(
y
CB
,
k
+
1
y
A
,
k
*
)
on the frequency domain symbol A and the concatenated symbol CB, where y A,k is the kth subcarrier of part A, and K is the total number of subcarriers in the orthogonal frequency division multiplexed preamble.
9. The method of claim 7 , wherein correcting the phase error due to the timing offset during a demodulation further comprises:
performing differential binary phase shift keying demodulation comprising correcting for the phase error using the estimated phase error.
10. The method of claim 1 , wherein N=1024.
11. The method of claim 1 , wherein K=0.
12. The method of claim 1 , wherein K is an integer greater than 0.
13. The method of claim 1 , wherein receiving the orthogonal frequency division multiplexed preamble symbol further comprises receiving over-the-air spread spectrum modulated radio frequency signals.
14. The method of claim 1 , wherein receiving the orthogonal frequency division multiplexed preamble symbol further comprises receiving digital video broadcast signals.
15. The method of claim 13 , wherein receiving over-the-air spread spectrum modulated radio frequency signals further comprises receiving radio frequency signals at an antenna.
16. The method of claim 2 , and further comprising:
performing differential binary phase shift keying demodulation on the transformed symbols A and CB in the frequency domain and extracting parameters to be utilized in receiving subsequent orthogonal frequency division multiplexed symbols.
17. The method of claim 16 , wherein extracting the parameters comprises extracting one or more modulated signal sequences.
18. An apparatus, comprising:
receiver circuitry configured to receive an orthogonal frequency division multiplexed preamble comprising symbol A of length N samples, a prefix C comprising a frequency shifted cyclic extension of a first portion of symbol A of length N/2 samples plus an offset K samples, a suffix B comprising a frequency shifted cyclic extension of a remaining portion of symbol A of length N/2 samples minus offset K samples;
concatenation circuitry configured to concatenate prefix C and suffix B to form a concatenated symbol CB of symbol A of length N samples; and
phase estimation circuitry configured to utilize the concatenated symbol CB to calculate at least one of a phase rotation estimate and a phase error.
19. The apparatus of claim 18 , and further comprising:
fast Fourier transform circuitry configured to perform a fast Fourier transform of length N on symbol A and a fast Fourier transform of length N on the concatenated symbol CB.
20. The apparatus of claim 19 , wherein the phase estimation circuitry configured to utilize the concatenated symbol CB to calculate at least one of a phase rotation estimate and a phase error further comprises:
phase rotation estimate circuitry configured to estimate a phase rotation due to an integer frequency offset; and
correction circuitry configured to perform a phase rotation correction using the phase rotation estimate.
21. The apparatus of claim 20 , wherein the phase rotation estimate circuitry further comprises circuitry configured to perform the calculation:
Δϕ
=
2
πɛ
N
C
N
where N C is the number of samples in prefix C, and ε is the normalized frequency offset.
22. The apparatus of claim 21 , wherein the correction circuitry configured to perform a phase rotation correction using the phase rotation estimate further comprises:
multiplier circuitry configured to perform a complex multiplication on the concatenated symbol CB to form a corrected symbol CB; and
adder circuitry configured to coherently add the symbol A transformed by the fast Fourier transform circuitry and the corrected symbol CB.
23. The apparatus of claim 19 , wherein the phase estimation circuitry configured to utilize the concatenated symbol CB, to calculate at least one of a phase rotation estimate and a phase error further comprises:
circuitry configured to estimate a phase error due to a timing offset; and
circuitry configured to correct the phase error in the frequency domain utilizing the phase error estimate due to the timing offset during a demodulation.
24. The apparatus of claim 23 , wherein the circuitry configured to estimate a phase error due to a timing offset further comprises circuitry configured to perform the calculation:
ϕ
^
=
1
K
∑
k
∈
CDS
arg
(
y
CB
,
k
+
1
y
A
,
k
*
)
on the frequency domain symbol A and the concatenated symbol CB, where y A,k is the kth subcarrier of part A, and K is the total number of subcarriers in the orthogonal frequency division multiplexed preamble.
25. The apparatus of claim 23 , wherein the circuitry configured to correct the phase error utilizing the phase error estimate due to the timing offset during a demodulation further comprises:
demodulation circuitry configured to correct the phase error due to the timing offset by performing differential binary phase shift keying demodulation including correcting for the phase error using the phase error estimate.
26. The apparatus of claim 18 , wherein the receiver circuitry further comprises:
analog front end circuitry configured to receive spread spectrum modulated radio frequency signals and to perform baseband processing on the received radio frequency signals; and
analog-to-digital circuitry configured to receive analog signals from the analog front end circuitry and to perform analog-to-digital conversion, outputting the orthogonal frequency division multiplexed preamble.
27. The apparatus of claim 18 , and further comprising an antenna.
28. A non-transitory computer readable medium containing instructions that, when executed by a programmable receiver processor, cause an apparatus to:
receive an orthogonal frequency division multiplexed preamble comprising symbol A of length N samples, a prefix C comprising a frequency shifted cyclic extension of a first portion of symbol A of length N/2 samples plus an offset K samples, a suffix B comprising a frequency shifted cyclic extension of a remaining portion of symbol A of length N/2 samples minus offset K samples;
concatenate prefix C and suffix B in the time domain to form a concatenated symbol CB that is a frequency shifted version of symbol A having length N; and
utilize the concatenated symbol CB to estimate at least one of a phase rotation and a phase error.
29. The non-transitory computer readable medium of claim 28 , further comprising instructions that, when executed by a programmable receiver processor, cause the apparatus to:
transform symbol A into the frequency domain by performing a fast Fourier transform of length N; and
transform the concatenated symbol CB into the frequency domain by performing a fast Fourier transform of length N.
30. The non-transitory computer readable medium of claim 29 , wherein the instructions, when executed by a programmable receiver processor, cause the apparatus to utilize the concatenated symbol CB to estimate at least one of a phase rotation and a phase error by calculating:
Δϕ
=
2
πɛ
N
C
N
where N C is the number of samples in C, and ε is the normalized frequency offset.
31. The non-transitory computer readable medium of claim 29 , wherein the instructions, when executed by a programmable receiver processor, cause the apparatus to:
correct the phase rotation due to an integer frequency offset by performing a complex multiplication on the concatenated symbol CB to form a corrected symbol CB; and
coherently add the symbol A transformed into the frequency domain and the corrected symbol CB.
32. The non-transitory computer readable medium of claim 29 , wherein the instructions, when executed by a programmable receiver processor, cause the apparatus to:
estimate phase error due to a timing offset; and
correct the phase error due to the timing offset during a demodulation.
33. The non-transitory computer readable medium of claim 32 , wherein the instructions, when executed by a programmable receiver processor, cause the apparatus to estimate the phase error by calculating:
ϕ
^
=
1
K
∑
k
∈
CDS
arg
(
y
CB
,
k
+
1
y
A
,
k
*
)
on the symbol A transformed into the frequency domain and the concatenated symbol CB, where y A,k is the kth subcarrier of part A, and K is the total number of subcarriers in the orthogonal frequency division multiplexed preamble.
34. The non-transitory computer readable medium of claim 33 , wherein the instructions, when executed by a programmable receiver processor, cause the apparatus to correct the phase error with:
differential binary phase shift keying demodulation including correcting for the phase error using the estimated phase error.
35. An apparatus, comprising:
an antenna for receiving over the air signals comprising orthogonal frequency division multiplexed symbols;
means for receiving an orthogonal frequency division multiplexed preamble comprising symbol A of length N samples from the antenna, a prefix C comprising a frequency shifted cyclic extension of a first portion of symbol A of length N/2 samples plus an offset K samples, and a suffix B comprising a frequency shifted cyclic extension of a remaining portion of symbol A of length N/2 samples minus offset K samples;
means for concatenating prefix C and suffix B in the time domain to form a concatenated symbol CB that is a frequency shifted version of symbol A having length N; and
means for utilizing the concatenated symbol CB to estimate at least one of a phase rotation and a phase error.Cited by (0)
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